Match Go CopyFile: sync EC volume journal to disk before copying

Go calls ecVolume.Sync() before copying EC volume files to ensure the
.ecj journal is flushed to disk. Added sync_to_disk() to EcVolume and
call it in the CopyFile EC branch.

seaweed-volume/src/server/grpc_server.rs

@@ -1215,6 +1215,14 @@ impl VolumeServer for VolumeGrpcService {
             file_name = v.file_name(&req.ext);
             drop(store);
         } else {
+            // Sync EC volume journal to disk before copying (matching Go's ecv.SyncToDisk())
+            {
+                let store = self.state.store.read().unwrap();
+                if let Some(ecv) = store.find_ec_volume(vid) {
+                    let _ = ecv.sync_to_disk();
+                }
+            }
+
             // EC volume: search disk locations for the file
             let store = self.state.store.read().unwrap();
             let mut found_path = None;

seaweed-volume/src/storage/erasure_coding/ec_volume.rs

@@ -169,6 +169,14 @@ impl EcVolume {
         format!("{}.ecj", self.idx_base_name())
     }
 
+    /// Sync the EC volume's journal file to disk (matching Go's ecv.SyncToDisk()).
+    pub fn sync_to_disk(&self) -> io::Result<()> {
+        if let Some(ref ecj_file) = self.ecj_file {
+            ecj_file.sync_all()?;
+        }
+        Ok(())
+    }
+
     // ---- Shard management ----
 
     /// Add a shard to this volume.